Pickup tube electron gun



May 3, 1966 n. L. SCHAEFER PICKUP TUBE ELECTRON GUN Filed Jan. 2. 1963FIG.|.

INVENTORI DONALD L. HAEFER HI TTORNEY.

3.249.790 PICKUP TUBE ELECTRON GUN Donald L. Schaefer, Schenectady,N.Y., assignor to General Electric Company, a corporation of New YorkFiled Jan. 2, 1963, Ser. No. 249,091

4 Claims. (Cl. 313-456) The. present invention relates to improvementsin picture signal generating tubes such as image orthicons, and moreparticularly to an improved electron gun construction for imageorthicons and the like" for reducing exposure of the photosensitiveelement of the tube to extraneous radiation.

A picture signal generating tube such as an image orthicon is providedwith a photosensitive cathode which acts to transduce an optical orlight radiation image to which it is exposed, to a pattern of electriccharge from which can be produced a suitable electrical readout signal.A problem in achieving optimum performance with such lig-htimage pick-uptubes, and particularly those intended to have enhanced sensitivity atlow light levels or enhanced sensitivity to radiation in the infraredregion of the spectrum, is that extraneous radiation from the thermioniccathode heater is transmitted, within the tube envelope itself, intoincidence on the photocathode or light-sensitive element of the tube,and thereby causes a false or extraneous electrical signal to begenerated. In general the effect of such extraneous radiation is toincrease the electrical noise output of the photocathode relative to thedesired signal, and thereby reduce the capability of the tube to detectand resolve weak signals such as those from scenes of low illumination.

A major portion of such extraneous radiation from the cathode heater hasbeen found to reach the photocathode by way of reflection from theinterior surface of the adjacent portion of the tube envelope, afterpassage through a heater support ceramic. The ceramic is usuallyselected for its mechanical and electrical properties and is frequentlymade of a translucent material such as aluminum oxide. Attempts to blocksuch extraneous cathode heater radiation from reaching the photocathodeby using heater support ceramics made more opaque by addition of a darkmaterial such as carbon have not proved satisfactory because additivessuch as carbon have a deleterious effect on the electrical properties ofsuch ceramics.

A principal object of the present invention, therefore, is to provide animproved cathode-heater construction for picture signal generating tubessuch as orthicons or vidicons, which minimize incidence of extraneousinternal radiation on the photocathode without deleteriously affectingthe electrical properties of the cathode-heater assembly.

This and other objects of the present invention will be apparent fromthe following description and the accompanying drawing wherein: 1

FIGURE 1 is apartially broken away fragmentary axial sectional view of apicture signal generating tube of the image orthicon type constructed inaccordance with the present invention; and

FIGURE 2 is an enlarged fragmentary axial sectional view of thethermionic cathode-heater assembly portion of the tube of FIGURE 1,constructed in accordance with the present invention;

Referring to FIGURE 1, the image orthicon there shown includes anevacuated envelope 2 closed at one end by a stem provided with a base 4having external leads 6. Envelope 2 also includes an image section 8having an image receiving window 10 on the interior of which issupported a photocathode 12 for converting optical image radiation to apattern of electric charge emitted from the back side of layer 12 andforming a stored ice charge image on a charge storage electrode 14, asis well known to those skilled in the art. Electrical readout of thecharge stored on electrode 14 is provided by an electron beam formedfrom electrons emitted from thermionic cathode 16, (FIG. 2) the electronbeam being suitably focused and deflected by means (not shown) externalto the tube envelope, and the current of reading beam electrons returnedfrom electrode v14 being suitably multiplied grade for 24 hours.

in electron multiplier dynode structure 18.

As best shown in FIGURE 2 the thermionic emissive material of thethermionic cathode 16 is supported on the metal end face 20 of acylindrical cathode sleeve 22. The cathode sleeve is in turn carried byan annular ceramic insulator 24 supported within a cylindrical cathodeand heater support casing 28. To facilitate support of some of dynodestructure 18, casing 28 has a coaxial cylindrical extension 29. Thesleeve 22 and insulator 24 and casing 28 are all arranged to be mountedwithin the envelope 2 coaxially withthe envelope axis 30. The rearwardend of cathode sleeve 22, that is the end facing the tube base 4, isopen so as to accommodate a coiled heater 32 which is carried by, andextends into'the sleeve 22 from a pair of relatively thick heater leads36. Leads 36 extend through and are supported in spaced relation by atransversely arranged ceramic insulative disc 40 contained within andclosing the rearward end of the casing 28.

The forward end of the casing 28 is completely closed by transversemetal electrode 50 .and control grid 52, except for the central openings54 and 56 provided in electrodes 50 and 52 respectively to permitpassage of electrons emitted from the cathode 16.

Since the forward end of the casing 28 is thus effectively closed byelectrodes 50, 52 to passage of extraneous radiation from the heater 32to the photocathode 12, it will be apparent that a principal source ofextraneous radiation from the thermionic heater 32, to which thephotocathode of the tube is sensitive, will be that emerging from therearward end of the casing 28 (or extension 29) and reflected from theinterior surface of the stern portion of the envelope and otherreflective surfaces within the tube back up to incidence on thephotocathode 12. In accordance with the present invention I make theinsulative support 40 opaque without undesirably affecting itselectrical properties by making the ceramic of a composition of aluminaand about 10% to 15% by weight of the oxides of chromium, cobalt, ormagnesium. The resulting ceramic is dark purple-red in color andtransmits substantially no light in the visible spectrum, and verylittle light in the infrared portion of the spectrum.

In the making of such ceramic supports 40 I compound a homogenousmixture of about to by weight alumina, and about 10% to 15% by weight ofone or a mixture of the oxides of chromium, cobalt, or magnesium. Thishomogeneous mixture is then pressed into shape with a suitable binder,such as carbo wax, and fired at a temperature of about 1400 to 1500degrees centi- The resultant ceramic has satisfactory mechanicalstrength, excellent electrical properties, and is substantiallycompletely opaque to extraneous radiation from a source such as heater32. A cathode-heater assembly such as shown in FIGURE 2, equipped withsuch a ceramic 40, is highly effective for preventing extraneousradiation from heater 32 from reaching photocathode 12, and thusminimizes generation of extraneous electrical signals resulting fromsuch radiation.

It will be appreciated by those skilled in the art that the inventionmay be carried out in various ways and may take various forms andembodiments other than the illustrative embodiments heretoforedescribed. Accordingly it is to be understood that the scope of theinvention is not limited by the details of the foregoing description,but will be defined in the following claims.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A cathode-heater assembly for a picture signal generating tube havinga photocathode comprising an opaque cathode-heater enclosure adapted tobe disposed in spaced relation to the photocathode and having anaperture opposite said photocathode, a cathode disposed within saidenclosure opposite said aperture and arranged to emit electrons throughsaid aperture toward said photocathode, a heater within said enclosureadjacent said cathode, said heater having supporting and energizingleads extending through an opening in said enclosure remote from saidaperture, a hard fired ceramic disc closing said opening and throughwhich said heater leads pass, said ceramic disc being composedessentially of a mixture of aluminum oxide and about to by weight ofmaterial from the group consisting of the oxides of cobalt, chromium,magnesium, and mixtures of such oxides whereby said ceramic disc isopaque to and prevents extraneous radiation from said heater frompassing out of said enclosure.

2. A cathode-heater assembly for a picture signal generating tube havinga photocathode comprising a cathodeheater enclosure having an opaquegenerally cylindrical portion adapted to be disposed in spaced relationto the photocathode, said enclosure cylindrical portion being closed atone end by an end wall forming an electrode having an aperture oppositesaid photocathode, a cathode disposed within said enclosure oppositesaid aperture and arranged to emit electrons through said aperturetoward said photocathode, a heater within said enclosure adjacent saidcathode, a hard fired ceramic disc forming a transverse closure for theother end of said cylindrical portion of said enclosure, said ceramicdisc being composed essentially of a mixture of aluminum oxide and about10% to 15% by weight of material from the group consisting of the oxidesof cobalt, chromium, magnesium, and mixtures of such oxides, wherebysaid ceramic disc is opaque to and blocks extraneous radiation from saidheater from passage out of said enclosure into incidence on thephotocathode, and supporting and energizing leads for said heaterextending through said ceramic disc.

3. A cathode-heater assembly for a picture signal generating tube havinga photocathode comprising a cylindrical metal casing adapted to bedisposed with its axis extending generally normal to the photocathode,means including an electrode forming a closure for the end of saidcasing adapted to face said photocathode, said electrode having acentral aperture, a hard fired ceramic discforming a transverse closurefor said casing axially spaced from said apertured electrode, a cathodeheater disposed within said casing between said apertured electrode andsaid ceramic disc and having supporting and energizing leads extendingseparately through said ceramic disc, said ceramic disc being composedessentially of a mixture of aluminum oxide and about 10% to 15% byWeight of material from the group consisting of the oxides of cobalt,chromium, magnesium, and mixtures of such oxides, and a thermionicemitter in said casing disposed in heat receiving relation with saidheater and having an electron emitting surface facing said aperturedelectrode.

4. In a picture signal generating tube having a photocathode fortransducing a light image to an electric charge pattern for reading byscanning with an electron beam, a cathode-heater assembly comprising alight-opaque casing adapted to be disposed in spaced relation with saidphotocathode and with one end facing said photocathode, means. includingan electrode entirely closing said one end of said casing, saidelectrode having a central aperture for passage of an electron beam, ahard fired ceramic disc forming a transverse closure for the other endof said casing, a cathode heater disposed Within said casing betweensaid electrode and said ceramic disc, energizing leads extending throughsaid ceramic disc in spaced apart insulated relationship and fixedlysupported by said disc, said ceramic disc being composed essentially ofa mixture of aluminum oxide and about 10% to 15% by weight of materialfrom the group consisting of the oxides of cobalt, chromium, magnesium,and mixtures of such oxides, and a thermionic emitter in said casingdisposed in heat receiving relation with said heater and having anelectron emitting surface facing the aperture of said aperturedelectrode, said thermionic emitter having leads connected to saidenergizing leads.

References Cited by the Examiner UNITED STATES PATENTS 2,015,172 9/1935Wiegand 313340 2,443,916 6/1948 Keiar 3 l382.1 2,540,621 2/1951. Johnson313-82.1 3,029,360 4/1962 Etter 313340 3,040,200 6/1962 Shaffer et al.313-82 X JOHN W. HUCKERT, Primary Examiner.

A. J. JAMES, Assistant Examiner.

1. A CATHODE-HEATER ASSEMBLY FOR A PICTURE SIGNAL GENERATING TUBE HAVINGA PHOTOCATHODE COMPRISING AN OPAQUE CATHODE-HEATER ENCLOSURE ADPATED TOBE DISPOSED IN SPACED RELATION TO THE PHOTOCATHODE AND HAVING ANAPERTURE OPPOSITE SAID PHOTOCATHODE, A CATHODE DISPOSED WITHIN SAIDENCLOSURE OPPOSITE SAID APERTURE AND ARRANGED TO EMIT ELECTRONS THROUGHSAID APERTURE TOWARD SAID PHOTOCATHODE, A HEATER WITHIN SAID ENCLOSUREADJACENT SAID CATHODE, SAID HEATER HAVING SUPPORTING AND ENERGIZINGLEADS EXTENDING THROUGH AN OPENING IN SAID ENCLOSURE REMOTE FROM SAIDAPERTURE, A HARD FIRED CERAMIC DISC CLOSING SAID OPENING AND THROUGHWHICH SAID HEATER LEADS PASS, SAID CERAMIC DISC BEING COMPOSEDESSENTIALLY OF A MIXTURE